Work vehicle and method for controlling same

ABSTRACT

A control unit in a work vehicle executes an automatic downshift for shifting a speed range of a transmission to the speed range at a lower speed than a current speed range. The control unit determines the execution of the automatic downshift on the basis of automatic downshift conditions. The automatic downshift conditions include whether the operating amount of the accelerator operating member is equal to or greater than a predetermined accelerator threshold, whether the vehicle speed is less than a predetermined speed threshold, and whether the acceleration is equal to or less than a predetermined acceleration threshold.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National stage application of InternationalApplication No. PCT/JP2015/050749, filed on Jan. 14, 2015. This U.S.National stage application claims priority under 35 U.S.C. §119(a) toJapanese Patent Application No. 2014-005026, filed in Japan on Jan. 15,2014, the entire contents of which are hereby incorporated herein byreference.

BACKGROUND

1. Field of the Invention

The present invention relates to a work vehicle and method forcontrolling the work vehicle.

2. Background Information

A work vehicle, such as a wheel loader, has an automatic downshiftfunction. When predetermined conditions are met, the automatic downshiftfunction enables the speed range of the transmission to be automaticallyshifted to a speed range lower than the current speed range.

In the work vehicle described in International Publication WO2008/120546, for example, the speed range is automatically shifted fromthe second speed or higher to the first speed when the hydraulicpressure of the boom cylinder is higher than the predetermined standardvalue, the height of the boom is lower than a predetermined height, andthe vehicle speed is equal to or less than a predetermined speed. As aresult, when a work vehicle traveling at the speed range at or above thesecond speed is carrying out excavating work, the speed range isautomatically shifted to the first speed.

SUMMARY

However, there is a problem that a delay in the execution of theautomatic downshift occurs with the determination method of theabove-mentioned automatic downshift based on the hydraulic pressure ofthe boom cylinder, the height of the boom, and the vehicle speed. Forexample, to prevent a false determination, the predetermined speed ispreferably a small value as possible. As a result, a state in which thevehicle speed falls due to the work vehicle plunging into a mound ofdirt when starting the excavating work can be determined with higheraccuracy. However, when the predetermined speed is set to a small value,the execution of the automatic downshift is not determined until thevehicle speed is low enough. As a result, a delay in the execution ofthe automatic downshift occurs.

An object of the present invention is to provide a work vehicle in whichthe false determination of an automatic downshift is suppressed and thedetermination can be made quickly, and a method for controlling the workvehicle.

A work vehicle according to a first exemplary embodiment of the presentinvention is equipped with an engine, a hydraulic pump, a workimplement, a travel device, a transmission, an accelerator operatingmember, an accelerator operating amount detecting unit, an vehicle speeddetecting unit, an acceleration detecting unit, and a control unit. Thehydraulic pump is driven by the engine. The work implement is driven byhydraulic fluid discharged from the hydraulic pump. The travel device isdriven by the engine. The transmission transmits driving power from theengine to the travel device. The accelerator operating amount detectingunit detects an operating amount of the accelerator operating member.The vehicle speed detecting unit detects the vehicle speed. Theacceleration detecting unit detects acceleration of the vehicle. Thecontrol unit executes an automatic downshift for shifting the speedrange of the transmission to a speed range at a lower speed than thecurrent speed range. The control unit determines the execution of theautomatic downshift on the basis of automatic downshift conditions. Theautomatic downshift conditions include the fact that the operatingamount of the accelerator operating member is equal to or greater than apredetermined accelerator threshold, the fact that the vehicle speed isless than a predetermined speed threshold, and the fact thatacceleration is equal to or less than a predetermined accelerationthreshold.

The execution of the automatic downshift is determined according to theoperating amount of the accelerator operating member and theacceleration of the vehicle as well as the vehicle speed in the workvehicle according to the present exemplary embodiment. As a result, thestate of the work vehicle can be determined with greater accuracy. Forexample, the fact that the acceleration of the vehicle is smallregardless of whether the accelerator operating member is being operatedto a large degree signifies that the work vehicle has plunged into adirt mound for performing excavating work, and the acceleration isinsufficient because the tractive force is insufficient. In this case,the automatic downshift can be executed appropriately in the workvehicle according to the present exemplary embodiment. Moreover, becausea false determination of the automatic downshift can be suppressed dueto the operating amount of the accelerator operating member and theacceleration of the vehicle, the speed threshold does not have to be setat an overly small value for preventing a false determination. As aresult, the automatic downshift determination can be made quickly.

The automatic downshift conditions preferably further include the factthat the work implement is in a predetermined work posture. In thiscase, the posture taken by the work implement during work in which aslow speed range is required is set to the predetermined work posturewhereby the execution of the automatic downshift can be determined withgreater accuracy.

The work vehicle preferably is further equipped with a brake devicedriven by hydraulic pressure. The automatic downshift conditions furtherinclude the fact that the pressure of the hydraulic fluid supplied tothe brake device is less than a predetermined brake threshold. In thiscase, the automatic downshift can be executed with even greater accuracydue to the detection that the operator does not intend to use the brakedevice.

The automatic downshift conditions preferably include the fact that theoperating amount of the accelerator operating member is equal to orgreater than a predetermined first accelerator threshold and theacceleration is a value corresponding to a deceleration. For example,when a value corresponding to the acceleration is positive, a valuecorresponding to the deceleration is negative. For example, when thepredetermined acceleration threshold is set to a negative value and theacceleration is equal to or less than the predetermined accelerationthreshold, the acceleration is determined as a value corresponding todeceleration. In this case, the automatic downshift is executed whilethe work vehicle is decelerating regardless of whether the acceleratoroperating member is being operated. This state signifies, for example,that the work vehicle is decelerating due to insufficient tractive forcewhen the work vehicle is beginning to plunge into the dirt mound withoutthe accelerator operating member being operated to a large degree. Inthis way, an appropriate state for executing the automatic downshift canbe determined with greater accuracy.

The automatic downshift conditions preferably include the fact that theoperating amount of the accelerator operating member is equal to orgreater than a second accelerator threshold that is greater than thefirst accelerator threshold and the acceleration is a valuecorresponding to an acceleration. In this case, the predeterminedacceleration threshold is set to a positive value for example. The factthat the acceleration is a value corresponding to an acceleration but isequal to or less than the predetermined acceleration threshold signifiesthat although the work vehicle is not decelerating, the work vehiclecannot accelerate sufficiently. Therefore, the automatic downshift isexecuted while the work vehicle is not accelerating sufficientlyregardless of whether the accelerator operating member is being operatedto a large degree. This state signifies, for example, that the workvehicle is not accelerating sufficiently due to insufficient tractiveforce when the work vehicle is plunging into the dirt mound regardlessof whether the accelerator operating member being operated to a largedegree. In this way, an appropriate state for executing the automaticdownshift can be determined with greater accuracy.

A control method for a work vehicle according to another exemplaryembodiment of the present invention is a control method for a workvehicle equipped with an engine, a hydraulic pump, a work implement, atravel device, a transmission, and an accelerator operating member. Thehydraulic pump is driven by the engine. The work implement is driven byhydraulic fluid discharged from the hydraulic pump. The travel device isdriven by the engine. The transmission transmits driving power from theengine to the travel device. The control method according to the presentexemplary embodiment includes first to fifth steps. The first stepinvolves detecting the operating amount of the accelerator operatingmember. The second step involves detecting the vehicle speed. The thirdstep involves detecting the acceleration of the vehicle. The fourth stepinvolves executing an automatic downshift for shifting the speed rangeof the transmission to a speed range at a lower speed than the currentspeed range. The fifth step involves determining the execution of theautomatic downshift on the basis of automatic downshift conditions. Theautomatic downshift conditions include the fact that the operatingamount of the accelerator operating member is equal to or greater than apredetermined accelerator threshold, the fact that the vehicle speed isless than a predetermined speed threshold, and the fact that theacceleration is equal to or less than a predetermined accelerationthreshold.

The execution of the automatic downshift is determined according to theoperating amount of the accelerator operating member and theacceleration of the vehicle as well as the vehicle speed in the controlmethod for the work vehicle according to the present exemplaryembodiment. As a result, the state of the work vehicle can be determinedwith greater accuracy. For example, the fact that the acceleration ofthe vehicle is small regardless of whether the accelerator operatingmember is being operated to a large degree signifies that although thework vehicle has plunged into a dirt mound for performing excavatingwork, the acceleration is insufficient because the tractive force isinsufficient. In this case, the automatic downshift can be executedappropriately in the work vehicle according to the present exemplaryembodiment. Moreover, because a false determination of the automaticdownshift can be suppressed due to the operating amount of theaccelerator operating member and the acceleration of the vehicle, thespeed threshold for preventing a false determination does not have to beset at an overly small value. As a result, the automatic downshiftdetermination can be made quickly.

In the work vehicle and the control method thereof according toexemplary embodiments of the present invention, a false determination ofthe automatic downshift can be suppressed and a determination can bemade quickly.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a side view of a work vehicle according to an exemplaryembodiment.

FIG. 2 is a schematic view of a drive system of the work vehicle.

FIG. 3 is a block diagram of a control system of the work vehicle.

FIG. 4 is a flow chart illustrating automatic downshift processing.

FIG. 5 illustrates first conditions of automatic downshift conditions.

FIG. 6 illustrates second conditions of the automatic downshiftconditions.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of the present invention will be explained indetail with reference to the drawings. FIG. 1 is a side view of a workvehicle 1 according to an exemplary embodiment of the present invention.As illustrated in FIG. 1, the work vehicle 1 is equipped with a vehiclebody frame 2, a work implement 3, traveling wheels 4 and 5, and anoperating cabin 6. The work vehicle 1 is a wheel loader and travels dueto the traveling wheels 4 and 5 being rotated and driven. The workvehicle 1 is able to carry out work, such as excavation, by using thework implement 3.

The work implement 3 and the traveling wheels 4 and 5 are attached tothe vehicle body frame 2. The work implement 3 is driven by hydraulicfluid from a below-mentioned first hydraulic pump 31 (see FIG. 2). Thework implement 3 has a boom 11 and a bucket 12. The boom 11 is mountedon the vehicle body frame 2. The work implement 3 has a boom cylinder 13and a bucket cylinder 14. The boom cylinder 13 and the bucket cylinder14 are hydraulic cylinders. One end of the boom cylinder 13 is attachedto the vehicle body frame 2. The other end of the boom cylinder 13 isattached to the boom 11. The boom 11 swings up and down due to theextension and contraction of the boom cylinder 13 due to hydraulic fluidfrom the first hydraulic pump 31. The bucket 12 is attached to the tipof the boom 11. One end of the bucket cylinder 14 is attached to thevehicle body frame 2. The other end of the bucket cylinder 14 isattached to the bucket 12 via a bell crank 15. The bucket 12 swings upand down due to the extension and contraction of the bucket cylinder 14with hydraulic fluid from the first hydraulic pump 31.

The operating cabin 6 is attached to the vehicle body frame 2. Theoperating cabin 6 is mounted on the vehicle body frame 2. A seat for theoperator, a below-mentioned operating device 50 (see FIG. 3) and so onare disposed in the operating cabin 6. The vehicle body frame 2 has afront frame 16 and a rear frame 17. The front frame 16 and the rearframe 17 are attached to each other in a manner that allows swinging inthe left-right direction.

The work vehicle 1 has a steering cylinder 18. The steering cylinder 18is attached to the front frame 16 and the rear frame 17. The steeringcylinder 18 is a hydraulic cylinder. The work vehicle 1 is able tochange the advancing direction to the right and left with the extensionand contraction of the steering cylinder 18 due to hydraulic fluid froma below-mentioned second hydraulic pump 32.

FIG. 2 is a schematic view of a drive system of the work vehicle 1. Asillustrated in FIG. 2, the work vehicle 1 has an engine 21, a powertake-off device 22 (referred to below as “PTO 22”), a transmission 23,and a travel device 24.

The engine 21 is, for example, a diesel engine. The output of the engine21 is controlled by adjusting the amount of fuel injected into thecylinders of the engine 21. The adjustment of the amount of fuel isconducted by a below-mentioned control unit 26 (see FIG. 3) controllinga fuel injection device 25 attached to the engine 21. The work vehicle 1is equipped with an engine rotation speed detecting unit 27. The enginerotation speed detecting unit 27 detects the engine rotation speed andtransmits a detection signal indicating the engine rotation speed to thecontrol unit 26.

The work vehicle 1 has the first hydraulic pump 31, the second hydraulicpump 32, and a third hydraulic pump 33. The PTO 22 transmits a portionof the driving power from the engine 21 to the hydraulic pumps 31, 32,and 33. That is, the PTO 22 distributes the driving power from theengine 21 to the hydraulic pumps 31, 32, and 33 and the transmission 23.

The first hydraulic pump 31 is driven by driving power from the engine21. Hydraulic fluid discharged from the first hydraulic pump 31 issupplied to the boom cylinder 13 and the bucket cylinder 14 through awork implement control valve 34.

The first hydraulic pump 31 is a variable displacement hydraulic pump.The discharge capacity of the first hydraulic pump 31 is changed bychanging the tilt angle of a skew plate or an inclined shaft of thefirst hydraulic pump 31. A first capacity control device 35 is connectedto the first hydraulic pump 31. The first capacity control device 35 iscontrolled by the control unit 26 and changes the tilt angle of thefirst hydraulic pump 31. As a result, the discharge flow rate of thefirst hydraulic pump 31 is controlled by the control unit 26.

The second hydraulic pump 32 is driven by driving power from the engine21. Hydraulic fluid discharged from the second hydraulic pump 32 issupplied to the above-mentioned steering cylinder 18 through a steeringcontrol valve 36.

The second hydraulic pump 32 is a variable displacement hydraulic pump.The discharge capacity of the second hydraulic pump 32 is changed bychanging the tilt angle of a skew plate or an inclined shaft of thesecond hydraulic pump 32. A second capacity control device 37 isconnected to the second hydraulic pump 32. The second capacity controldevice 37 is controlled by the control unit 26 and changes the tiltangle of the second hydraulic pump 32. As a result, the dischargecapacity of the second hydraulic pump 32 is controlled by the controlunit 26.

The third hydraulic pump 33 is driven by driving power from the engine21. The hydraulic fluid discharged from the third hydraulic pump 33 issupplied to a brake device 39 via a brake control valve 38. The thirdhydraulic pump 33 is a variable displacement hydraulic pump. Thedischarge capacity of the third hydraulic pump 33 is changed by changingthe tilt angle of a skew plate or an inclined shaft of the thirdhydraulic pump 33. A third capacity control device 40 is connected tothe third hydraulic pump 33. The third capacity control device 40 iscontrolled by the control unit 26 and changes the tilt angle of thethird hydraulic pump 33. As a result, the discharge flow rate of thethird hydraulic pump 33 is controlled by the control unit 26.

The PTO 22 transmits a portion of the driving power from the engine 21to the transmission 23. The transmission 23 transmits the driving powerfrom the engine 21 to the travel device 24. The transmission 23 changesspeeds and outputs the driving power from the engine 21.

The transmission 23 is, for example, an electric-mechanical transmission(EMT) having a planetary gear mechanism and an electric motor connectedto a rotating element of the planetary gear mechanism. Alternatively,the transmission 23 may be a hydraulic-mechanical transmission (HMT)having a planetary gear mechanism and a hydraulic motor connected to arotating element of the planetary gear mechanism. The speed ratio of thetransmission 23 can be continuously changed by controlling the motor inthe EMT or the HMT. Alternatively, the Transmission 23 may be a torqueconverter transmission having a torque converter and a multi-stage speedchanging device. Alternatively, the transmission 23 may be a hydrostatictransmission (HST).

The travel device 24 has an axle 41 and the traveling wheels 4 and 5.The axle 41 transmits driving power from the transmission 23 to thetraveling wheels 4 and 5. Consequently the traveling wheels 4 and 5rotate. The work vehicle 1 is equipped with a vehicle speed detectingunit 42. The vehicle speed detecting unit 42 detects the rotation speed(referred to below as “output rotation speed”) of an output shaft of thetransmission 23. The output rotation speed corresponds to the vehiclespeed and consequently the vehicle speed detecting unit 42 detects thevehicle speed by detecting the output rotation speed. The vehicle speeddetecting unit 42 detects the rotating direction of the output shaft ofthe transmission 23. The rotating direction of the output shaftcorresponds to the traveling direction of the work vehicle 1 andconsequently the vehicle speed detecting unit 42 functions as a traveldirection detecting unit that detects the traveling direction of thework vehicle 1 by detecting the rotating direction of the output shaft.The vehicle speed detecting unit 42 transmits detection signalsindicating the output rotation speed and the rotating direction to thecontrol unit 26.

FIG. 3 is a block diagram of a control system provided in the workvehicle 1. As illustrated in FIG. 3, the work vehicle 1 has theoperating device 50 and the control unit 26. The operating device 50 isoperated by the operator. The operating device 50 has an acceleratoroperating device 51, a work implement operating device 52, a speedchange operating device 53, a forward-reverse travel operating device 54(referred to below as “FR operating device 54”), a steering operatingdevice 55, a brake operating device 56, and a setting device 57.

The accelerator operating device 51 has an accelerator operating member51 a and an accelerator operation detecting unit 51 b. The acceleratoroperating member 51 a is operated to set a target rotation speed of theengine 21. The rotation speed of the engine 21 is changed due to theaccelerator operating member 51 a being operated. The acceleratoroperation detecting unit 51 b detects an operating amount (referred tobelow as “accelerator operating amount”) of the accelerator operatingmember 51 a. The accelerator operation detecting unit 51 b transmits adetection signal indicating the accelerator operating amount to thecontrol unit 26.

The work implement operating device 52 has a work implement operatingmember 52 a and a work implement operation detecting unit 52 b. The workimplement operating member 52 a is operated to actuate the workimplement 3. The work implement operation detecting unit 52 b detects aposition of the work implement operating member 52 a. The work implementoperation detecting unit 52 b outputs a detection signal indicating theposition of the work implement operating member 52 a to the control unit26. The work implement operation detecting unit 52 b detects anoperating amount (referred to below as “work implement operatingamount”) of the work implement operating member 52 a by detecting theposition of the work implement operating member 52 a.

The speed change operating device 53 has a speed change operating member53 a and a speed change operation detecting unit 53 b. The operator isable to select a speed range of the transmission 23 by operating thespeed change operating member 53 a. The speed change operation detectingmember 53 b detects the position of the speed change operating member 53a. The position of the speed change operating member 53 a corresponds toa plurality of speed ranges, such as a first speed and a second speedand the like. For example, a speed range from a first speed to a fourthspeed can be selected with the transmission 23 of the present exemplaryembodiment. The speed change operation detecting member 53 b outputs adetection signal indicating the position of the speed change operatingmember 53 a to the control unit 26.

The speed change operating device 53 further includes a kick-down switch53 c. The speed change operation detecting member 53 b outputs adetection signal indicating the fact that the kick-down switch 53 c hasbeen operated to the control unit 26.

The FR operating device 54 has a forward/reverse travel operating member54 a (referred to below as “FR operating member 54 a”) and aforward/reverse travel position detecting unit 54 b (referred to belowas “FR position detecting unit 54 b”). The operator can switch betweenforward and reverse travel of the work vehicle 1 by operating the FRoperating member 54 a. The FR operating member 54 a is selectivelyswitched between a forward travel position (F), a neutral position (N),and a reverse travel position (R). The FR position detecting unit 54 bdetects the position of the FR operating member 54 a. The FR positiondetecting unit 54 b outputs a detection signal indicating the positionof the FR operating member 54 a to the control unit 26.

The steering operating device 55 has a steering operating member 55 a.The steering operating device 55 drives the steering control valve 36 bysupplying pilot hydraulic pressure based on an operation of the steeringoperating member 55 a to the steering control valve 36. The steeringoperating device 55 may drive the steering control valve 36 byconverting an operation of the steering operating member 55 a to anelectrical signal. The operator is able to change the travel directionof the work vehicle 1 to the right or left by operating the steeringoperating member 55 a.

The brake operating device 56 has a brake operating member 56 a and abrake operation detecting unit 56 b. The operator is able to operate thebraking force of the work vehicle 1 by operating the brake operatingmember 56 a. The brake operation detecting unit 56 b detects anoperating amount (referred to below as “brake operating amount”) of thebrake operating member 56 a. The brake operation detecting unit 56 boutputs a detection signal indicating the brake operating amount to thecontrol unit 26.

The setting device 57 is a device for enabling various settings of thework vehicle 1. The setting device 57 is a touch panel-type displayinput device for example. Alternatively, the setting device 57 may be adevice provided with hardware keys and a display. The setting device 57outputs an input signal indicating an input setting to the control unit26. The setting device 57 also displays various types of information ofthe work vehicle 1 in response to a command signal from the control unit26.

The work vehicle 1 has a boom position detecting unit 61 and a bucketposition detecting unit 62. The boom position detecting unit 61 detectsa position of the boom 11. For example, the boom position detecting unit61 detects a position of the boom 11 by detecting the angle of the boom11. The boom position detecting unit 61 may be a sensor for directlydetecting the angle of the boom 11. Alternatively, the boom positiondetecting unit 61 may detect the angle of the boom 11 by detecting astroke amount of the boom cylinder 13. The boom position detecting unit61 outputs a detection signal indicating the position of the boom 11 tothe control unit 26.

The bucket position detecting unit 62 detects the position of the bucket12. For example, the bucket position detecting unit 62 detects theposition of the bucket 12 by detecting the angle of the bucket 12. Thebucket position detecting unit 62 may also be a sensor for directlydetecting the angle of the bucket 12. Alternatively, the bucket positiondetecting unit 62 may detect the angle of the bucket 12 by detecting thestroke amount of the bucket cylinder 14. The bucket position detectingunit 62 outputs a detection signal indicating the position of the bucket12 to the control unit 26.

The work vehicle 1 has a boom pressure detecting unit 63. The boompressure detecting unit 63 detects a bottom pressure of the boomcylinder 13. The bottom pressure of the boom cylinder 13 is the pressureof the hydraulic fluid inside the oil chamber at the bottom side of theboom cylinder 13. When the boom cylinder 13 extends, hydraulic fluid issupplied to the oil chamber at the bottom side of the boom cylinder 13.When the boom cylinder 13 contracts, hydraulic fluid is discharged fromthe oil chamber at the bottom side of the boom cylinder 13. When theboom 11 is in a holding state, a hydraulic pressure corresponding to aload for holding the boom 11 acts on the oil chamber at the bottom sideof the boom cylinder 13. The boom pressure detecting unit 63 inputs adetection signal indicating the bottom pressure of the boom cylinder 13to the control unit 26.

The work vehicle 1 has a brake pressure detecting unit 64. The brakepressure detecting unit 64 detects the brake pressure. The brakepressure is the pressure of the hydraulic fluid supplied to the brakedevice 39. The brake pressure detecting unit 64 inputs a detectionsignal indicating the brake pressure to the control unit 26.

The control unit 26 has a calculation device, such as a centralprocessing unit (CPU), and a memory, such as a RAM or a ROM, andconducts processing for controlling the work vehicle 1. For example, thecontrol unit 26 transmits a command signal indicating a command throttlevalue to the fuel injection device 25 to achieve the target rotationspeed of the engine 21 in response to the accelerator operating amount.The control unit 26 controls the hydraulic pressure supplied to the boomcylinder 13 and the bucket cylinder 14 by controlling the work implementcontrol valve 34 on the basis of the detection signal from the workimplement operation detecting unit 52 b. As a result, the boom cylinder13 and the bucket cylinder 14 expand and contract to operate the workimplement 3. The control unit 26 controls the hydraulic pressuresupplied to the brake device 39 in response to the brake operatingamount. As a result, the braking force from the brake device 39 isadjusted.

Moreover, the control unit 26 controls the transmission 23 on the basisof the detection signals from the various detecting units. For example,the control unit 26 switches the rotating direction of the output shaftof the transmission 23 in response to the position of the FR operatingdevice 54. The control unit 26 switches the speed range of thetransmission 23 in response to the position of the speed changeoperating member 53 a. When the kick-down switch 53 c is operated, thecontrol unit 26 shifts the speed range of the transmission 23 to thefirst speed.

The control unit 26 executes the automatic downshift when thepredetermined automatic downshift conditions are met. The automaticdownshift signifies shifting the speed range of the transmission 23 downto the first speed. Processing pertaining to the automatic downshiftwill be discussed next.

FIG. 4 is a flow chart illustrating automatic downshift processing. Asillustrated in FIG. 4, the speed range of the transmission 23 is set tothe speed range selected by the speed change operating member 53 a instep S101. It is assumed here that the speed range is set to the secondspeed or higher.

In step S102, a determination is made as to whether the automaticdownshift conditions meet first conditions. FIG. 5 illustrates the firstconditions. As illustrated in FIG. 5, the first conditions include thefact that the automatic downshift setting is valid (condition a 1). Whenthe automatic downshift setting is set by the setting device 57 to avalid setting, the automatic downshift setting is determined as valid.

The first conditions include the fact that the FR operating member 54 ais in the forward travel position (condition a 2), the fact that thebrake pressure is less than a predetermined brake threshold Pth_br(condition a 3), the fact that deceleration has continued at least for apredetermined first time period threshold Ta1 (condition a 4), the factthat the vehicle speed is less than a predetermined first speedthreshold Vth1 (condition a 5), and the fact that the acceleratoroperating amount is greater than a predetermined first acceleratorthreshold Ath1 (condition a 6).

As illustrated in FIG. 3, the control unit 26 has an accelerationdetecting unit 26 a. The acceleration detecting unit 26 a calculates theacceleration of the work vehicle 1 from the vehicle speed detected bythe vehicle speed detecting unit 42. When an acceleration sensor isprovided in the work vehicle 1, the acceleration sensor may detect theacceleration of the work vehicle 1 as an acceleration detecting unit.When the acceleration of the work vehicle 1 is equal to or less than apredetermined first acceleration threshold, the work vehicle 1 isdetermined as decelerating. The first acceleration threshold is anegative value.

The first conditions include the fact that the forward travel hascontinued for at least a first forward travel time period threshold Tb1and that the work implement 3 is in an automatic downshift posture(condition a 7). When the position of the boom 11 is near the ground andthe bucket 12 is horizontal, the work implement 3 is determined as beingin the automatic downshift posture. The position of the boom 11 beingnear the ground signifies that the position of the boom 11 detected bythe boom position detecting unit 61 is within a predetermined heightrange that corresponds to being near the ground. The bucket 12 beinghorizontal signifies that the angle of the bucket 12 is within an anglerange where the bottom surface of the bucket 12 is substantiallyhorizontal as illustrated in FIG. 1.

The first conditions include the fact that the work vehicle 1 is in anexcavating state (condition a 8). When the height of the boom 11 isequal to or less than a predetermined height threshold and the bottompressure of the boom cylinder 13 is equal to or greater than apredetermined pressure threshold, the work vehicle 1 is determined asbeing in the excavating state.

When the above-mentioned (condition a 1) and the (condition a 2) and the(condition a 3) and the (condition a 4) and the (condition a 5) and the(condition a 6) and the (condition a 7 or condition a 8) are met, it isdetermined that the first conditions are met. When the first conditionsare met in step S102 in FIG. 4, the speed range of the transmission 23is shifted down to the first speed in step S103. That is, the automaticdownshift is executed. The routine advances to step S104 when the firstconditions are not met in the step S102.

In step S104, a determination is made as to whether the secondconditions of the automatic downshift conditions are met. FIG. 6illustrates the second conditions. As illustrated in FIG. 6, the secondconditions include the fact that the automatic downshift setting isvalid (condition b 1), the fact that the FR operating member 54 a is inthe forward travel position (condition b 2), and the fact that the brakepressure is less than the predetermined brake threshold Pth_br(condition b 3). The conditions b 1 to b 3 are the same as theabove-mentioned conditions a 1 to a 3 of the first conditions.

The second conditions further include the fact a lack of accelerationhas continued for at least a predetermined second time period thresholdTa 2 (condition b 4), the fact that the vehicle speed is less than apredetermined second speed threshold Vth2 (condition b 5), and the factthat the accelerator operating amount is greater than a predeterminedsecond accelerator threshold Ath2 (condition b 6). The insufficientacceleration signifies that the acceleration is zero or greater andequal to or less than the predetermined second acceleration threshold.The second acceleration threshold is a positive value. The second timeperiod threshold Ta 2 is greater than the first time period thresholdTaof the first conditions. The second speed threshold Vth2 is less thanthe first speed threshold Vth1 of the first conditions. The secondaccelerator threshold Ath2 is greater than the first acceleratorthreshold Ath1 of the first conditions.

The second conditions include the fact that the forward travel hascontinued for at least a second forward travel time period threshold Tb2 and that the work implement 3 is in the automatic downshift posture(condition b 7), and the fact that the work vehicle 1 is in theexcavating state (condition b 8). The second forward travel time periodthreshold Tb 2 is less than the first forward travel time periodthreshold Tb 1 of the first conditions.

When the above-mentioned (condition b 1) and the (condition b 2) and the(condition b 3) and the (condition b 4) and the (condition b 5) and the(condition b 6) and (the condition b 7 or the condition b 8) are met, itis determined that the second conditions are met. When the secondconditions are met in step S104 in FIG. 4, the speed range of thetransmission 23 is shifted down to the first speed in step S103. Thatis, the automatic downshift is executed. The routine returns to stepS101 if the second conditions are not met in step S104. That is, thespeed range of the transmission 23 is maintained at the speed rangeselected by the speed change operating member 53 a.

In step S105, it is determined whether the position of the speed changeoperating member 53 a is in the neutral position or the reverse travelposition. The routine returns to step S101 if the operating position ofthe speed change operating member 53 a is in the neutral position or thereverse travel position. Therefore, the routine returns to step S101 ifthe position of the speed change operating member 53 a is changed fromthe forward travel position to the neutral position or the reversetravel position.

In step S106, it is determined as to whether the vehicle speed is equalto or greater than a predetermined release speed. The release speed isgreater than the above-mentioned first vehicle speed threshold Vth1. Therelease speed is greater than the above-mentioned second vehicle speedthreshold Vth2. When the vehicle speed is equal to or greater than therelease speed, the routine returns to step S101.

As described above, the execution of the automatic downshift isdetermined according to the accelerator operating amount and theacceleration of the work vehicle 1 as well as the vehicle speed in thework vehicle 1 according to the present exemplary embodiment. As aresult, the state of the work vehicle 1 can be determined with greateraccuracy.

Specifically, the first conditions include the fact that thedeceleration has continued at least for a predetermined first timeperiod threshold Ta1 (condition a 4), the fact that the vehicle speed isless than a predetermined first speed threshold Vth1 (condition a 5),and the fact that the accelerator operating amount is greater than apredetermined first accelerator threshold Ath1 (condition a 6). Thisstate is, for example, a state in which the work vehicle 1 isdecelerating due to insufficient tractive force when the work vehicle 1is beginning to plunge into the dirt mound without the acceleratoroperating member 51 a being operated to a large degree. This type ofstate can be determined accurately and the automatic downshift can beexecuted in the work vehicle 1 according to the present exemplaryembodiment.

The second conditions include the fact that insufficient accelerationhas continued for at least a predetermined second time period thresholdTa2 (condition b 4), the fact that the vehicle speed is less than thepredetermined second speed threshold Vth2 (condition b 5), and the factthat the accelerator operating amount is greater than the predeterminedsecond accelerator threshold Ath2 (condition b 6). This state is, forexample, a state in which the work vehicle 1 is plunging into the dirtmound and the work vehicle 1 is not accelerating sufficiently due toinsufficient tractive force regardless of whether the acceleratoroperating member 53 a is being operated to a large degree. This type ofstate can be determined accurately and the automatic downshift can beexecuted in the work vehicle 1 according to the present exemplaryembodiment.

Moreover, because the false determination of the automatic downshift canbe suppressed by considering the accelerator operating amount and theacceleration of the work vehicle 1, the speed thresholds Vth1 and Vth2for preventing false determinations do not have to be set at overlysmall values. As a result, the automatic downshift determination can bemade quickly.

Although an exemplary embodiment of the present invention has beendescribed so far, the present invention is not limited to the aboveexemplary embodiments and various modifications may be made within thescope of the invention.

The present invention is not limited to the above-mentioned wheel loaderand may be applied to another type of work vehicle, such as a bulldozer,a tractor, a forklift, or a motor grader.

The automatic downshift may include shifting down to a speed range otherthan the first speed. For example, the automatic downshift may includeshifting down from the current speed range to a speed range that is onespeed stage lower.

A portion of the conditions included in the above-mentioned firstconditions and the second conditions may be omitted or changed.Alternatively, another condition may be added to the first conditionsand the second conditions.

The speed range of the transmission is not limited to the first speed tothe fourth speed. For example, the speed range of the transmission maybe from the first speed to the third speed. Alternatively, the speedrange of the transmission may be from the first speed to a speed equalto or greater than the fourth speed.

A work vehicle and a method for controlling the same can be provided inwhich a false determination of an automatic downshift can be suppressedand the determination can be made quickly according to exemplaryembodiments of the present invention.

1. A work vehicle comprising: an engine; a hydraulic pump driven by theengine; a work implement driven by hydraulic fluid discharged from thehydraulic pump; a travel device driven by the engine; a transmissionthat transmits a driving power from the engine to the travel device; anaccelerator operating member; an accelerator operation amount detectingunit that detects an operating amount of the accelerator operatingmember; a vehicle speed detecting unit that detects a vehicle speed; anacceleration detecting unit that detects acceleration of the vehicle;and a control unit configured to execute an automatic downshift forshifting a speed range of the transmission to a speed range at a lowerspeed than a current speed range; the control unit determining theexecution of the automatic downshift on the basis of automatic downshiftconditions including whether the operating amount of the acceleratoroperating member is equal to or greater than a predetermined acceleratorthreshold, whether the vehicle speed is less than a predetermined speedthreshold, and whether the acceleration is equal to or less than apredetermined acceleration threshold.
 2. The work vehicle according toclaim 1 wherein the automatic downshift conditions further includewhether the work implement is in a predetermined work posture.
 3. Thework vehicle according to claim 1, further comprising a brake devicedriven by hydraulic pressure, the automatic downshift conditions furtherinclude whether the hydraulic pressure of hydraulic fluid supplied tothe brake device is less than a predetermined brake threshold.
 4. Thework vehicle according to claim 1, wherein the automatic downshiftconditions include whether the operating amount of the acceleratoroperating member is equal to or greater than a predetermined firstaccelerator threshold and the acceleration is a value corresponding to adeceleration.
 5. The work vehicle according to claim 4, wherein theautomatic downshift conditions include whether the operating amount ofthe accelerator operating member is equal to or greater than apredetermined second accelerator threshold that is greater than thefirst accelerator threshold and the acceleration is a valuecorresponding to an acceleration.
 6. A control method for a work vehicleincluding an engine, a hydraulic pump driven by the engine, a workimplement driven by hydraulic fluid discharged from the hydraulic pump,a travel device driven by the engine, a transmission that transmits adriving power from the engine to the travel device, and an acceleratoroperating member, the method comprising: a step for detecting anoperating amount of the accelerator operating member; a step fordetecting a vehicle speed; a step for detecting an acceleration of thevehicle; a step for executing an automatic downshift for shifting aspeed range of the transmission to a speed range at a lower speed than acurrent speed range; and a step for determining the execution of theautomatic downshift on the basis of automatic downshift conditionsincluding whether the operating amount of the accelerator operatingmember is equal to or greater than a predetermined acceleratorthreshold, whether the vehicle speed is less than a predetermined speedthreshold, and whether the acceleration is equal to or less than apredetermined acceleration threshold.
 7. The work vehicle according toclaim 2, further comprising a brake device driven by hydraulic pressure,the automatic downshift conditions further include whether the hydraulicpressure of hydraulic fluid supplied to the brake device is less than apredetermined brake threshold.
 8. The work vehicle according to claim 7,wherein the automatic downshift conditions include whether the operatingamount of the accelerator operating member is equal to or greater than apredetermined first accelerator threshold and the acceleration is avalue corresponding to a deceleration.
 9. The work vehicle according toclaim 8, wherein the automatic downshift conditions include whether theoperating amount of the accelerator operating member is equal to orgreater than a predetermined second accelerator threshold that isgreater than the first accelerator threshold and the acceleration is avalue corresponding to an acceleration.